In interference current therapy, an electromedical treatment process is employed in which, in the area of treatment in question, a stimulus is developed with a low-frequency electric current. The immediate use of low-frequency alternating current of sufficient current intensity runs into difficulties however, because the electrical resistance of the area of treatment is relatively high at low frequencies. Because of this high resistance, there are disagreeable stimulations of the skin in the region of attachment of the electrodes when using low frequency signals, because of the necessary high voltage, and the intensities of stimulation deep within the treatment area are inadequate.
Therefore, in interference current therapy, two (or more) medium-frequency currents (with a frequency for example of a few thousand Hertz) that are independent of each other are applied by means of electrodes placed on the skin. These medium-frequency currents, which are respectively ineffective for stimulation, present frequencies that differ by a low-frequency difference. These medium-frequency currents penetrate into the depth of the area of treatment so as to interfere with each other and thereby produce a low-frequency beat that corresponds to the frequency difference. This low beat frequency, because of the physiological properties of the nerves, is stimulating.
In known arrangements of this kind (see German Pat. No. 1,764,672), use is made of two frequencies of about 5000 Hz, where one differs slightly from the other, e.g. 5000 Hz and 5001 Hz. The great difficulty in this arrangement resides in the fact that this slight difference in frequency is very difficult to maintain, and also that one of the two frequencies has to be variable.